EMC: Eta "Tip of the Month"

Updated: 11 Feb 1999

Cold bias in 2-m air temperatures over snowcover in Eta model.

Ken Mitchell (Kenneth.Mitchell@noaa.gov)

Snow cover effects are an important influence on both daytime and nighttime 2-meter air temperatures in the atmosphere. NCEP/EMC has and continues to make a concerted effort in the Eta model to both properly a) initialize snow cover and snow depth fields and b) model the difficult physics of snow cover effects on the lower atmosphere. We are joined in this effort by key collaborators in the Office of Hydrology (OH) and NESDIS.

We are keenly aware of a 2-meter air temperature cold bias over snow cover in the present operational Eta model. Additionally, we are rapidly getting a handle on the physical causes of the bias and are well along on testing new extensions to the Eta model land-surface physics to eliminate the bulk of this bias. However, operational implementation of these improvements will most likely not take place until Fall of 1999, because adequate testing of the changes has been hampered by a) the current saturation of our CRAY computer mainframes , b) Y2K tests, and c) resources devoted to getting NCEP models running on the new Class VIII IBM/SP Supercomputer being delivered and installed during the current winter.

The 2-m cold bias in the Eta over snow cover has rather different causes, we believe, during daytime and nighttime, so I am going to address them separately.

Daytime Cold Bias

During the day, if temperature and radiation conditions are such that the snow pack physics begin melting the snow, then the snow surface skin temperature is constrained to the freezing point (0 C) throughout the snowmelt process. Hence the overlying 2-m air temperatures in the model rarely rise above about 1-2 C during this melt process. This is in fact reasonable over melting DEEP snowpack, but not over shallow melting snowpack. In the real world, over shallow melting snowpack (say less than about 3-4 inches), the snowpack becomes patchy, with spotty bare ground patches breaking through, and the area-average skin temperature can rise notably above freezing, with overlying 2-m air temperatures going well above freezing (say upper 30's F, 40's F, or even low 50's F in extemely favorable conditions in late spring with clear skies, relatively strong solar radiation, and warm southerly advection). The present physics of the Eta model (and NGM as well) does NOT allow for patchiness in the snow cover. Even for very shallow snowdepth, the Eta model (and NGM) assumes the entire grid box is 100 percent snow covered.

Hence, the surface skin temperature is held to 0 C over melting snow until ALL the snow cover is melted in the model. One can easily observe this model behavior in the Eta model BUFR output, by inspecting the skin temperature, 2-m air temperature, and snowdepth (expressed as water equivalent). When the melting snowdepth reaches zero, say, during a mid-day melt episode, the skin temperature and 2-m air temperature finally begin to rise significantly above freezing.

A secondary problem that contributes to the above is that we feel that our modeled surface albedo over snow cover may be too high, thus reflecting too much surface solar insolation. We reduced the albedo over snow somewhat in operational Eta changes we made in Feb 97, but we feel more reduction may be needed.

Another contributing secondary problem is the physical modeling of ground heat flux (or subsurface heat flux) under the snowpack. This heat flux process under snowpack is difficult to model and is very sensitive to the snowdepth and snowpack density. Often the current physics yields too much ground heat flux, and hence less energy is available to melt the snow and heat the lower atmosphere.

NCEP/EMC and the Office of Hydrology have formulated new snowpack physics to address the above problems, and we have tested the new physics extensively in "uncoupled" mode (wherein the entire land/snow/soil/vegetation physics subsystem is executed separate from the Eta model, using observed low-level atmospheric conditions). A formal journal paper describing these uncoupled tests has recently been accepted for publication (entitled "A parameterization of snowpack and frozen ground intended for use in NCEP Weather and Climate Models", by Victor Koren, John Schaake, Ken Mitchell, and co-authors, to appear in Journal of Geophysical Research). Testing of the new physics in the Eta model will begin next month, but operational implementation will very likely await next Fall.

Aside comment: We have heard some folks wrongly state that the surface skin temperature in the Eta model is always 0 C. This is only true in the melting state. In colder nonmelt conditions, the Eta skin temperature over snow is below freezing, even well below freezing if air temperatures warrant.

Nighttime Cold Bias

The nighttime 2-m temperature cold bias in the Eta model is a more general problem, that occurs over both non-snow covered and snow-covered ground, both in the warm season and the cool season.

This bias is worse in the cool season, and worse yet in the cool season over snow cover. The basic cause of this cool bias is that the Eta model physics yields too little near-surface vertical turbulent mixing during calm nighttime conditions (i.e. stable nigttime low-level temperature inversions, referred to as the stable boundary layer). This problem is greater in the cool season because of the longer nights, the greater tendency for cool season nighttime winds to go calm, and the cooling effect of snow cover yielding even stronger nighttime temperature inversions. Hence, the following conditions tend to yield the greatest cold bias in nighttime 2-meter temperatures: calm winds, clear skies, long night, and snow cover. The nighttime situation has a positive feedback character, because as the low-level inversion sets in, the surface vertical turbulent mixing of heat falls off, which in turn acts to strengthen the inversion, etc. The snow cover exacerbates the feedback, because the snowpack insulates the lower atmosphere from the deep soil heat reservoir, which in the absence of snow can act as a heat source to the lower atmosphere at night.

Ironically, as the vertical resolution of a model increases (i.e. Eta versus NGM or AVN), we feel there is a tendency for the above positive feedback to be enhanced, because the enhanced resolution is able to better resolve stronger, more extreme, shallow nighttime inversions.

Hence, we are embarking on tests of different approaches to the process of nighttime, near-surface, vertical turbulent mixing, to account for such real-world subgrid effects as slope breezes and horizontal turbulent mixing that enhance nighttime turbulent mixing.

Initialization of Snowcover

In an extensive collaboration with NESDIS, NCEP/EMC has invested much effort toward improving the initialization of snow cover in all NCEP NWP models. As a result of this collaboration, in Jan 98, NESDIS began operational production of a new daily, 23-km N. Hemisphere snow cover and sea-ice analysis (known as the Interactive Multi-sensor Snow -- IMS) in the SAB branch of NESDIS. The Eta model began operationally using this analysis on 03 Jun 98. The AVN and NGM models began using the IMS product during the Fall of 98. The IMS is only a snow cover product thus far (not snow depth). So the Eta, NGM, and AVN models continue to use the daily Air Force snowdepth product as the source of snowdepth, but the snow cover of the latter is screened to completely agree with the new daily NESDIS product. (As a future improvement, we are working on incorporating the thrice-weekly 1-km snow-water equivalent analysis available from NOHRSC (NWS/OH) during the months of Jan-May.)

Initial snow analysis errors can obviously be a factor in the cold biases discussed earlier. In particular, if the snow cover analysis has snow present at a particular location, when in fact no snow cover exists, then the cited cold bias tendencies are particularly notable. Forecasters should utilize BUFR output inspection tools (e.g. BUFKIT) to examine the initial Eta snowdepth at their station in a given Eta run. If the Eta is initialized with snow at their location, but no snow or shallow snow exists there in reality, then the forecaster should expect the Eta 2-m temperature forecast to be too cold, especially during daytime hours, and especially if the daytime temperatures are otherwise expected to be above freezing.

Forecasters can also inspect the daily NESDIS IMS snowcover product online by visiting the NESDIS website. (if necessary, scroll to snow analysis section)

Feedback on the NESDIS IMS snow/ice product can be given to the IMS Analyst Team Leader, Tom Baldwin at Thomas.Baldwin@noaa.gov

The IMS is updated once daily around 5 pm EST each day, and first gets into the 00Z Eta run.